Sewage treatment in general involves the use of clarifiers or settling tanks into which liquefied sewage is introduced. The solid matter suspended in the liquid settles to the bottom of the tank as sludge, and the top clarified. This clarified liquid is then introduced into a second tank so that there may be further precipitation of the solid matter, with a consequent further clarification of the liquid. Subsequent settling tanks continue to clear the liquid and collect the solid matter as sludge. Typical of such settling tanks with sludge collecting mechanisms are those shown in U.S. Pat. Nos. 1,951,462 of Wing, 2,122,383 of Scott, and 3,764,012 of Bohuke et al. The Bohuke et al apparatus is of interest in that the settling tank is divided into several compartments which perform the settling and clarification functions of a plurality of tanks.
Removal of the clarified effluent is generally accomplished by means of an annular trough formed by weirs which permit the clarified liquid to spill into the trough from which it is subsequently removed by suitable pipes or conduits. Such weir and trough arrangements are shown, for example, in U.S. Pat. Nos. 2,052,676 of York and 2,236,128 of Poole.
In settlement systems in general, and especially in such systems where a plurality of settling tanks are used, the liquid effluent still contains, after the first or second settling operation, large quantities of organic matter suspended therein. One consequence of the presence of such organic matter is the formulation of a scum which floats on the liquid and resists settling to the bottom. This scum can be removed by apparatuses such as that shown in U.S. Pat. No. 3,338,419 of Smith, which uses oar shaped paddles to impel the scum into collection pipes through which it is drained away.
Another, more serious, consequence of the presence of organic matter in the effluent is that such matter promotes the growth of algae at a rapid rate. Such algae builds up on the weir surfaces and on the surfaces of the settling tank itself. Such a buildup can result in clogging of the weir passages and the drain conduits of the system, as well as adding additional contaminants to the effluent. In practice it has become necessary to scrub down the system periodically to remove accumulated algae and, in some cases, to shut down the system, drain the tanks, and scrub them out. Such procedures are time consuming, labor intensive, expensive, and unpleasant for those who have to scrub the tanks.
While it is virtually impossible to prevent the formation of algae, it is highly desirable that such algae formation and growth be controlled to the extent that the frequency of system shut down is materially reduced.